Added metadata to compiled modules that record whether instruction counting instrumentation is present.

Made lucetc more flexible in its interpretation of the LD environment variable. It now accepts a space-separated set of tokens; the first token specifies the program to invoke, and the remaining tokens specifying arguments to be passed to that program. Thanks, @froydnj!

Added public LucetcOpt methods to configure the canonicalize_nans setting. Thanks, @roman-kashitsyn!

Fixed lucet-runtime's use of CPUID to not look for extended features unless required by the module being loaded, avoiding a failure on older CPUs where that CPUID leaf is not present. Thanks, @shravanrn!